Digital cameras often use what are referred to as rolling shutters. In the case of a rolling shutter different lines of a sensor are read out at different times. Exposure control is often controlled in rolling shutter embodiments by zeroing or emptying the pixel buffers of a row at a point in time that precedes the readout time by the desired exposure time. In this way the pixel value is a function of the amount of light captured during the desired exposure period before readout.
Since in rolling shutter implementations, the exposure period of different lines will occur at different time periods based on the order the lines are read out, the pixel values of different lines correspond to different time periods. In the case of motion, given that lines of a sensor are exposed at different times the same object may appear at different locations in the captured image depending on when a line exposed and read out. Often sensors are read out sequentially from top to bottom when using a rolling shutter. In the case of an image captured by a single sensor which reads lines out sequentially, the moving object may appear in the image as a blur or streak.
Some camera devices include multiple sensors. In the case where a camera device includes multiple sensors, some of them with overlapping fields of view, different sensors may capture the same scene area at different times. In the case of motion, if the same scene area is captured at different times by different sensors, depending on the rate of motion, the same object may appear at different locations in the images captured by different sensors. This problem may be particularly noticeable in the case where sensors correspond to camera modules of different focal lengths given that the use of different focal lengths often result in different size areas of a scene being captured by different sensors. In the case of sensors of the same size and number of pixel rows corresponding to different focal length camera modules, if the rows of pixel values are read out from the sensors from top to bottom, different portions of a scene area are likely to be captured by different sensors at different times. This can make combining of images captured by different camera modules, e.g., corresponding to camera modules having different focal lengths, difficult to combine particularly in the case of motion. This is because different sensors may capture the same scene area at different times and thus may capture different scene content. For example, in the case of motion due to capture of the same scene area at different times by different sensors, a moving ball may appear in an image captured by one sensor at a different location than in an image captured by another sensor.
In view of the above discussion, it should be appreciated that there is a need for methods and/or apparatus for controlling sensor readout of different sensors of an apparatus in a way that results in different sensors capturing the same scene area at or near the same time. It would be desirable if such methods and/or apparatus could be used with rolling shutters. In this way differences between images captured by different sensors that are controlled using rolling shutters, due to the capture of different scene areas at different times, can be kept small or avoided.
While not necessary for all embodiments, it would be desirable if in at least some embodiments the readout rate of one or more sensors is not slowed down as compared to the read out rate of other sensors corresponding to camera modules with a different focal length for purposes of synchronizing with other sensors in the same device.